The present invention relates to a method and apparatus for measuring the axle load of a running vehicle and, particularly, to such a method and apparatus capable of performing computation in a sufficiently short period by virtue of a small amount of computation needed.
An apparatus is known which determines the weight of a running vehicle by measuring vertical forces (hereinafter referred to as axle loads) that are imparted to the road surface by the respective axles of the vehicle and summing up the measured axle loads.
By the way, while running, a vehicle vibrates at resonance frequencies corresponding to the entire body and respective portions due to impacts that are caused by asperity of a road, acceleration, etc. Therefore, the instantaneous vertical force imparted to the road surface by an axle of a running vehicle varies as shown in FIG. 7. In FIG. 7, the thin line indicates a waveform corresponding to a main vibration of a running vehicle and the thick line indicates a waveform corresponding to a combined vibration of the main vibration and an auxiliary vibration of the running vehicle. The main vibration means a resonance of the entire vehicle and the auxiliary vibration means a resonance that occurs at a portion of the vehicle or a load.
Thus, a waveform as shown in FIG. 7 is obtained when axle load meters are arranged along a road and outputs of those axle load meters are picked up upon passage of a vehicle. Conventionally, the axle load of a running vehicle is determined from such a waveform according to the following methods.
1) Wave components of an output of an axle load meter is dealt with as errors. More specifically, the axle load of a running vehicle is measured by an axle load meter installed at a selected measuring location where vibration of a running vehicle is as small as possible. The midpoint of vibration is directly determined from outputs of the axle load meter, and employed as an axle load measurement value of the running vehicle.
2) Axle load meters are stalled at a plurality of measuring locations. A main vibration waveform corresponding to a resonance frequency of the entire vehicle and an auxiliary vibration waveform produced by a resonance at a portion of the vehicle are estimated from a partial waveform obtained by those axle load meters. The midpoint of vibration is determined from the estimated main vibration waveform and auxiliary vibration waveform, and employed as an axle load measurement value of the running vehicle.
3) To simplify method 2), a main vibration and an auxiliary vibration are estimated with an assumption that the frequency of the main vibration is equal to or around 3 Hz. The midpoint of a resulting vibration is determined and employed as an axle load measurement value of the running vehicle.
However, in method 1), in which a wave portion of an output of the axle load meter is dealt with as errors, the measurement accuracy is low even if the measurement by the axle load meter is performed at a location where vibration of a running vehicle is as small as possible. Further, method 1) has a problem that the axle load meter installation point is restricted.
In method 2), in which a main vibration waveform corresponding to a resonance frequency of the entire vehicle and an auxiliary vibration waveform produced by a resonance at a portion of the vehicle are estimated from a partial waveform obtained by the axle load meters, the amount of computation becomes enormous and it is therefore difficult to complete the computation in a required time even with the use of a high-speed computer. Where vehicles pass successively, the "required time" means a time from a time point when the first vehicle passes a measuring point to a time point when the second vehicle passes it.
Further, in method 2), the axle load meters are required to have high measurement accuracy. In method 2), a vibration waveform of a vehicle is computed, estimated and reproduced based on a partial waveform measured by the axle load meters. Therefore, if the accuracy of the axle load meters is low, measurement errors of the axle load meters are amplified to prevent a correct measurement.
In method 3), the frequency of a main vibration is assumed to be equal to or around 3 Hz. Therefore, although method 3) is suitable for measurements on large-sized vehicles having a main vibration whose frequency is equal to or around 3 Hz, it may cause a large error for other types of vehicles having a main vibration whose frequency is not in that range. In addition, the reliability of measurement data is low because not all large-sized vehicles have a main vibration whose frequency is equal to or around 3 Hz.